Distillation is generally used in the separation of gas composition containing a plurality of components. The distillation is the procedure of concentrating a particular component in a mixture through the use of the difference in vapor pressure among individual component substances. As a mixture to be distilled is heated, individual components gradually evaporate from the surface of the solution, and boiling starts when the sum of the vapor pressures of the individual components agrees with the pressure of the system. The composition of vapors that emanate during this period almost depends on both of the composition of components on the surface of the solution and the vapor pressures (partial pressures) of the individual components at the temperature, according to the Raoult's law. A batch-type method and a continuous distillation method are known as industrial distillation methods.
The description above is the case where a reaction does not occur between components to be separated, and the case where a reaction occurs between gas components, between liquid-phase components, or between gas-liquid phases takes complicated evaporation behaviors.
For example, conventionally, in the case where equilibrium in an equilibrium reaction is disadvantageous to a product side, enhancing reaction efficiency (equilibrium conversion) by separating at least one type of products from the reaction system to make the equilibrium advantageous to the product side is generally performed. Although various methods are known as methods for separating products from the reaction system, distillation separation is one of the most generally performed methods. A method of pursuing a reaction by shifting the equilibrium reaction toward the product side with products removed from the reaction system by distillation is called reactive distillation, and an explanation about the reactive distillation is described in, for example, Non Patent Literature 1 by showing specific examples.
In general, the reactive distillation is carried out by using a distillation column such as a continuous multi-stage distillation column (reactive distillation apparatus). While higher-boiling-point components contained in a reaction solution become mostly distributed on the lower stage side of the distillation column along with the progress of the reaction in performing reactive distillation within the distillation column, lower-boiling-point components become mostly distributed on the upper stage side of the distillation column. Thus, in the distillation column, the internal temperature (solution temperature) decreases with movement from the bottom of the column toward the top of the column. The lower the temperature is, the lower the reaction rate of the equilibrium reaction becomes. Therefore, the reaction rate lowers with movement from the bottom of the column toward the top of the column in performing reactive distillation within the distillation column. Specifically, the reaction efficiency of the equilibrium reaction decreases with movement from the bottom of the column toward the top of the column in performing reactive distillation within the distillation column.
Thus, further increasing a temperature within the column has been studied in order to more improve reaction efficiency, i.e., to more accelerate the reaction rate, and a method of advantageously pursuing a reaction by supplying a solvent to a reactive distillation column and increasing a temperature within the reactive distillation column is disclosed in, for example, in Patent Literature 1, as a method for efficiently performing an equilibrium reaction represented by raw material (P)+raw material (Q)product (R)+product (S), especially, a transesterification reaction.
On the other hand, in the distillation separation of products in a system in which an equilibrium reaction represented by raw material (P)product (R)+product (S) exists, distillation is difficult in the case where the reaction rate is higher in the right-to-left direction (i.e., in reverse reaction) than in the left-to-right direction at a distillation separation temperature. In such a reaction, equilibrium may tilt toward the right side (product side) in a high-temperature region, and the case where distillation separation is influenced by other side reactions or the like is frequent in a high-temperature region though there is also a possibility of distillation separation. For example, it is not preferable to apply the method as described above to, for example, the distillation separation of a mixture containing an active hydrogen-containing compound and a compound that reversibly reacts with the active hydrogen-containing compound, especially, the distillation separation of a thermally decomposable product such as an N-substituted carbamic acid ester, an N-substituted thiocarbamic acid ester, or an N-substituted dithiocarbamic acid ester. For example, the case of thermal decomposition of the N-substituted carbamic acid ester is based on the following reason:
It has been known since long ago that an isocyanate and a hydroxy compound are obtained by the thermal decomposition of the N-substituted carbamic acid ester (see e.g., Non Patent Literature 2). The basic reaction of thermal decomposition of the N-substituted carbamic acid ester is illustrated by the following formula:
whereinR represents an a-valent organic residue; R′ represents a monovalent organic residue; and a represents an integer of 1 or larger.
The thermal decomposition reaction represented by the formula is reversible, and its equilibrium tilts at low temperatures toward the left-hand side where an N-substituted carbamic acid ester forms, and by contrast, tilts at high temperatures toward the right-hand side where an isocyanate and a hydroxy compound form.
Meanwhile, the N-substituted carbamic acid ester tends to be accompanied with various irreversible side reactions such as unfavorable thermal denaturation reactions and the condensation reaction of the isocyanate that forms by the thermal decomposition of the N-substituted carbamic acid ester. Examples of the side reactions include a reaction forming a urea bond represented by, for example, formula (2), a reaction forming carbodiimides represented by, for example, formula (3), and a reaction forming isocyanurates represented by, for example, formula (4):
wherein R represents an aliphatic group or an aromatic group.
Particularly, in the case where the N-substituted carbamic acid ester is an N-substituted polycarbamic acid ester, a plurality of groups in one molecule may cause the side reactions as described above to form a high-molecular-weight form. Thus, it is impossible to solve these problems only by increasing a temperature within the distillation column using, for example, the method of Patent Literature 1 described above.
In the production of an isocyanate by the thermal decomposition of an N-substituted carbamic acid ester, a method of rapidly separating reaction products or decreasing the formation of by-products by dilution with an inactive solvent has been devised for reducing the formation of by-products capable of forming deposits in a reactor.
A method of using a reactor in a thin film form or tube form to thermally decompose an N-substituted carbamic acid ester in the presence of an inactive solvent is disclosed in, for example, Patent Literature 2 and Patent Literature 3. A method of using a reaction column to thermally decompose an N-substituted carbamic acid ester in the presence of an inactive solvent is disclosed in Patent Literature 4.
Moreover, a method of using a reactive rectifying column to thermally decompose an N-substituted carbamic acid ester in the presence of a particular inactive solvent and at the same time, separate an isocyanate and an alcohol that form is described in Patent Literature 5 and Patent Literature 6.